Capcom just announced at TGS that the Dead Rising 2: Case Zero prologue sold over 500,000 copies in under 2 weeks. With that knowledge, one can understand why they'd do an epilogue such as: Dead Rising 2: Case West.
Keiji Inafune revealed that the downloadable episode will take place after the events of Dead Rising 2 and, as teased, will feature Frank West (of the original Dead Rising) working with latest protaganist Chuck Greene. Inafune concluded, "Apologies to PS3 fans, but this too will be an Xbox 360 exclusive." We'll update as we learn more.